r"""File-like objects that read from or write to a string buffer. This implements (nearly) all stdio methods. f = StringIO()      # ready for writingf = StringIO(buf)   # ready for readingf.close()           # explicitly release resources heldflag = f.isatty()   # always falsepos = f.tell()      # get current positionf.seek(pos)         # set current positionf.seek(pos, mode)   # mode 0: absolute; 1: relative; 2: relative to EOFbuf = f.read()      # read until EOFbuf = f.read(n)     # read up to n bytesbuf = f.readline()  # read until end of line ('\n') or EOFlist = f.readlines()# list of f.readline() results until EOFf.truncate([size])  # truncate file at to at most size (default: current pos)f.write(buf)        # write at current positionf.writelines(list)  # for line in list: f.write(line)f.getvalue()        # return whole file's contents as a string Notes:- Using a real file is often faster (but less convenient).- There's also a much faster implementation in C, called cStringIO, but  it's not subclassable.- fileno() is left unimplemented so that code which uses it triggers  an exception early.- Seeking far beyond EOF and then writing will insert real null  bytes that occupy space in the buffer.- There's a simple test set (see end of this file)."""try:    from errno import EINVALexcept ImportError:    EINVAL = 22 __all__ = ["StringIO"] def _complain_ifclosed(closed):    if closed:        raise ValueError, "I/O operation on closed file" class StringIO:    """class StringIO([buffer])     When a StringIO object is created, it can be initialized to an existing    string by passing the string to the constructor. If no string is given,    the StringIO will start empty.     The StringIO object can accept either Unicode or 8-bit strings, but    mixing the two may take some care. If both are used, 8-bit strings that    cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause    a UnicodeError to be raised when getvalue() is called.    """    def __init__(self, buf = ''):        # Force self.buf to be a string or unicode        if not isinstance(buf, basestring):            buf = str(buf)        self.buf = buf        self.len = len(buf)        self.buflist = []        self.pos = 0        self.closed = False        self.softspace = 0     def __iter__(self):        return self     def next(self):        """A file object is its own iterator, for example iter(f) returns f        (unless f is closed). When a file is used as an iterator, typically        in a for loop (for example, for line in f: print line), the next()        method is called repeatedly. This method returns the next input line,        or raises StopIteration when EOF is hit.        """        _complain_ifclosed(self.closed)        r = self.readline()        if not r:            raise StopIteration        return r     def close(self):        """Free the memory buffer.        """        if not self.closed:            self.closed = True            del self.buf, self.pos     def isatty(self):        """Returns False because StringIO objects are not connected to a        tty-like device.        """        _complain_ifclosed(self.closed)        return False     def seek(self, pos, mode = 0):        """Set the file's current position.         The mode argument is optional and defaults to 0 (absolute file        positioning); other values are 1 (seek relative to the current        position) and 2 (seek relative to the file's end).         There is no return value.        """        _complain_ifclosed(self.closed)        if self.buflist:            self.buf += ''.join(self.buflist)            self.buflist = []        if mode == 1:            pos += self.pos        elif mode == 2:            pos += self.len        self.pos = max(0, pos)     def tell(self):        """Return the file's current position."""        _complain_ifclosed(self.closed)        return self.pos     def read(self, n = -1):        """Read at most size bytes from the file        (less if the read hits EOF before obtaining size bytes).         If the size argument is negative or omitted, read all data until EOF        is reached. The bytes are returned as a string object. An empty        string is returned when EOF is encountered immediately.        """        _complain_ifclosed(self.closed)        if self.buflist:            self.buf += ''.join(self.buflist)            self.buflist = []        if n < 0:            newpos = self.len        else:            newpos = min(self.pos+n, self.len)        r = self.buf[self.pos:newpos]        self.pos = newpos        return r     def readline(self, length=None):        r"""Read one entire line from the file.         A trailing newline character is kept in the string (but may be absent        when a file ends with an incomplete line). If the size argument is        present and non-negative, it is a maximum byte count (including the        trailing newline) and an incomplete line may be returned.         An empty string is returned only when EOF is encountered immediately.         Note: Unlike stdio's fgets(), the returned string contains null        characters ('\0') if they occurred in the input.        """        _complain_ifclosed(self.closed)        if self.buflist:            self.buf += ''.join(self.buflist)            self.buflist = []        i = self.buf.find('\n', self.pos)        if i < 0:            newpos = self.len        else:            newpos = i+1        if length is not None:            if self.pos + length < newpos:                newpos = self.pos + length        r = self.buf[self.pos:newpos]        self.pos = newpos        return r     def readlines(self, sizehint = 0):        """Read until EOF using readline() and return a list containing the        lines thus read.         If the optional sizehint argument is present, instead of reading up        to EOF, whole lines totalling approximately sizehint bytes (or more        to accommodate a final whole line).        """        total = 0        lines = []        line = self.readline()        while line:            lines.append(line)            total += len(line)            if 0 < sizehint <= total:                break            line = self.readline()        return lines     def truncate(self, size=None):        """Truncate the file's size.         If the optional size argument is present, the file is truncated to        (at most) that size. The size defaults to the current position.        The current file position is not changed unless the position        is beyond the new file size.         If the specified size exceeds the file's current size, the        file remains unchanged.        """        _complain_ifclosed(self.closed)        if size is None:            size = self.pos        elif size < 0:            raise IOError(EINVAL, "Negative size not allowed")        elif size < self.pos:            self.pos = size        self.buf = self.getvalue()[:size]        self.len = size     def write(self, s):        """Write a string to the file.         There is no return value.        """        _complain_ifclosed(self.closed)        if not s: return        # Force s to be a string or unicode        if not isinstance(s, basestring):            s = str(s)        spos = self.pos        slen = self.len        if spos == slen:            self.buflist.append(s)            self.len = self.pos = spos + len(s)            return        if spos > slen:            self.buflist.append('\0'*(spos - slen))            slen = spos        newpos = spos + len(s)        if spos < slen:            if self.buflist:                self.buf += ''.join(self.buflist)            self.buflist = [self.buf[:spos], s, self.buf[newpos:]]            self.buf = ''            if newpos > slen:                slen = newpos        else:            self.buflist.append(s)            slen = newpos        self.len = slen        self.pos = newpos     def writelines(self, iterable):        """Write a sequence of strings to the file. The sequence can be any        iterable object producing strings, typically a list of strings. There        is no return value.         (The name is intended to match readlines(); writelines() does not add        line separators.)        """        write = self.write        for line in iterable:            write(line)     def flush(self):        """Flush the internal buffer        """        _complain_ifclosed(self.closed)     def getvalue(self):        """        Retrieve the entire contents of the "file" at any time before        the StringIO object's close() method is called.         The StringIO object can accept either Unicode or 8-bit strings,        but mixing the two may take some care. If both are used, 8-bit        strings that cannot be interpreted as 7-bit ASCII (that use the        8th bit) will cause a UnicodeError to be raised when getvalue()        is called.        """        if self.buflist:            self.buf += ''.join(self.buflist)            self.buflist = []        return self.buf  # A little test suite def test():    import sys    if sys.argv[1:]:        file = sys.argv[1]    else:        file = '/etc/passwd'    lines = open(file, 'r').readlines()    text = open(file, 'r').read()    f = StringIO()    for line in lines[:-2]:        f.write(line)    f.writelines(lines[-2:])    if f.getvalue() != text:        raise RuntimeError, 'write failed'    length = f.tell()    print 'File length =', length    f.seek(len(lines[0]))    f.write(lines[1])    f.seek(0)    print 'First line =', repr(f.readline())    print 'Position =', f.tell()    line = f.readline()    print 'Second line =', repr(line)    f.seek(-len(line), 1)    line2 = f.read(len(line))    if line != line2:        raise RuntimeError, 'bad result after seek back'    f.seek(len(line2), 1)    list = f.readlines()    line = list[-1]    f.seek(f.tell() - len(line))    line2 = f.read()    if line != line2:        raise RuntimeError, 'bad result after seek back from EOF'    print 'Read', len(list), 'more lines'    print 'File length =', f.tell()    if f.tell() != length:        raise RuntimeError, 'bad length'    f.truncate(length/2)    f.seek(0, 2)    print 'Truncated length =', f.tell()    if f.tell() != length/2:        raise RuntimeError, 'truncate did not adjust length'    f.close() if __name__ == '__main__':    test()